A GUNMAN wearing a gas mask killed at least 12 people when he opened fire at a Colorado cinema on the opening night of the latest Batman movie.

At least 50 others were injured, police in Aurora have confirmed.

The gunman was arrested soon after and named as local man James Holmes, 24.

FBI agents and police discovered Holmes' apartment was booby trapped. Authorities evacuated five buildings as they determined how to disarm flammable and explosive material.

It was the worst mass shooting in the US since an Army psychiatrist killed 13 soldiers and civilians at Fort Hood, Texas, in 2009.

Police break into booby-trapped home of the killer

And it brought memories of the massacre at nearby Columbine High School in 1999, where two students opened fire and killed 12 classmates and a teacher.

Some movie-goers said they thought today's attack was part of The Dark Knight Rises, one of the most highly anticipated films of the summer. Then they saw a silhouette of a person in the smoke at the front of the theatre, pointing a gun at the crowd.

"There were bullet casings just falling on my head. They were burning my forehead," Jennifer Seeger said, adding that the gunman, dressed like a SWAT team member, fired steadily except when he stopped to reload.

"Every few seconds it was just boom, boom, boom," Seeger said. "He would reload and shoot and anyone who would try to leave would just get killed."

Holmes was arrested near a car outside the multiplex theatre in Aurora. The FBI said there was no indication the shooting was tied to any terrorist groups.

Report of the killings at point when death toll was thought to be 14

The Pentagon said some military members were either killed or wounded. Aurora is home to a large Defence Department satellite intelligence operation at Buckley Air Force Base.

Holmes had an assault rifle, a shotgun and two pistols. He was a student at the University of Colorado School of Medicine in Denver but dropped out last month, spokeswoman Jacque Montgomery said. She did not know when he started school or why he withdrew.

Police released a written statement from Holmes' family: "Our hearts go out to those who were involved in this tragedy and to the families and friends of those involved."

Some of the injured at the theatre were children, with the youngest a four-month-old baby who has been released from treatment. Victims were being treated for chemical exposure apparently related to canisters thrown by the gunman.

The Dark Knight Rises opened across the world today. But the shooting prompted officials to cancel the Paris premiere, with workers pulling down the red carpet display at a theatre on the Champs-Elysees. Two police officers were stationed outside the AMC theatre in New York's Times Square.

Police officer on guard at scene of shooting

"Warner Bros. and the filmmakers are deeply saddened to learn about this shocking incident. We extend our sincere sympathies to the families and loved ones of the victims at this tragic time," the studio said.

President Barack Obama said he was saddened by the "horrific and tragic shooting" and he cut short campaigning to return to the White House.

Movie-goers spoke of their terror as violence erupted.

The gunman released a gas that smelled like pepper spray from a green canister with a tag on it, Ms Seeger said.

"I thought it was showmanship. I didn't think it was real," she said.

Ms Seeger said she was in the second row when the gunman pointed a gun at her face. At first, "I was just a deer in headlights. I didn't know what to do," she said. Then she ducked to the ground as the gunman shot people seated behind her.

The cinema in Aurora where Batman film was showing

She said she began crawling toward an exit when she saw a girl about 14 years old "lying lifeless on the stairs." She saw a man with a bullet wound in his back and tried to check his pulse, but "I had to go. I was going to get shot."

Witness Shayla Roeder said she saw a young teenage girl on the ground bleeding outside the theatre. "She just had this horrible look in her eyes .... We made eye contact and I could tell she was not all right," Ms Roeder said.

The violent and chaotic scene erupted about 12.30 am as a gunman stood at the front of one of the Century 16 theatres at the Aurora Mall.

Police chief Dan Oates said: "Witnesses tell us he released some sort of canister. They heard a hissing sound and some gas emerged and the gunman opened fire."

Police, ambulances and emergency crews rushed to the scene after frantic calls started flooding the 911 switchboard.